Positive Magnetoresistance Effect Research Articles

Unconventional room-temperature negative magnetoresistance effect in Au/n-Ge:Sb/Au devices

Non-magnetic semiconductor materials and their devices have attracted wide attention since they are usually prone to exhibit large positive magnetoresistance (MR) effect in a low static magnetic field environment at room temperature. However, how to obtain a large room-temperature negative MR effect in them remains to be studied. In this paper, by designing an Au/n-Ge:Sb/Au device with metal electrodes located on identical side, we observe an obvious room-temperature negative MR effect in a specific 50 T pulsed high magnetic field direction environment, but not in a static low magnetic field environment. Through the analysis of the experimental measurement of the Hall effect results and bipolar transport theory, we propose that this unconventional negative MR effect is mainly related to the charge accumulation on the surface of the device under the modulation of the stronger Lorentz force provided by the pulsed high magnetic field. This theoretical analytical model is further confirmed by regulating the geometry size of the device. Our work sheds light on the development of novel magnetic sensing, magnetic logic and other devices based on non-magnetic semiconductors operating in pulsed high magnetic field environment.

Room-temperature magnetoresistance in Ni78Fe22/C8-BTBT/Ni78Fe22 nanojunctions fabricated from magnetic thin-film edges using a novel technique.

Molecular spintronic devices are gaining popularity because the organic semiconductors with long spin relaxation times are expected to have long spin diffusion lengths. A typical molecular spintronic device consists of organic molecules sandwiched between two magnetic layers, which exhibits magnetoresistance (MR) effect. Nanosized devices are also expected to have a high spin polarization, leading to a large MR effect owing to effective orbital hybridization. However, most studies on nanosized molecular spintronic devices have investigated the MR effect at low temperatures because of the difficulty in observing the MR effect at room temperature. Here we focus on high-mobility molecules expected to show long spin diffusion lengths, which lead to the observation of the MR effect in nanoscale junctions at room temperature. In this study, we fabricate magnetic nanojunctions consisting of high-mobility molecules, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), sandwiched between two Ni78Fe22 thin films with crossed edges. Transmission electron microscopy (TEM) images reveal that C8-BTBT molecular layers with smooth and clear interfaces can be deposited on the Ni78Fe22 thin-film edges. Consequently, we observe a clear positive MR effect, that is, R P < R AP, where R P and R AP are the resistances in the parallel (P) and antiparallel (AP) configurations, respectively, of two magnetic electrodes in the Ni78Fe22/C8-BTBT/Ni78Fe22 nanojunctions at room temperature. The obtained results indicate that the spin signal through the C8-BTBT molecules can be successfully observed. The study presented herein provides a novel nanofabrication technique and opens up new opportunities for research in high-mobility molecular nano-spintronics.

Influence of Device Geometry on Transport Properties of Topological Insulator MicroflakesSupported by the National Natural Science Foundation of China (Grant No. 11961141011), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB28000000), and the National Key Research and Development Program of China (Grant No. 2016YFA0300600).

In the transport studies of topological insulators, microflakes exfoliated from bulk single crystals are often used because of the convenience in sample preparation and the accessibility to high carrier mobilities. Here, based on finite element analysis, we show that for the non-Hall-bar shaped topological insulator samples, the measured four-point resistances can be substantially modified by the sample geometry, bulk and surface resistivities, and magnetic field. Geometry correction factors must be introduced for accurately converting the four-point resistances to the longitudinal resistivity and Hall resistivity. The magnetic field dependence of inhomogeneous current density distribution can lead to pronounced positive magnetoresistance and nonlinear Hall effect that would not exist in the samples of ideal Hall bar geometry.

Structure, Morphology, Heat Capacity, and Electrical Transport Properties of Ti3(Al,Si)C2 Materials.

A study of Ti3Al1−xSixC2 (x = 0 to x = 1) MAX-phase alloys is reported. The materials were obtained from mixtures of Ti3AlC2 and Ti3SiC2 powders with hot pressing sintering technique. They were characterised with X-ray diffraction, heat capacity, electrical resistivity, and magnetoresistance measurements. The results show a good quality crystal structure and metallic properties with high residual resistivity. The resistivity weakly varies with Si doping and shows a small, positive magnetoresistance effect. The magnetoresistance exhibits a quadratic dependence on the magnetic field, which indicates a dominant contribution from open electronic orbits. The Debye temperatures and Sommerfeld coefficient values derived from specific heat data show slight variations with Si content, with decreasing tendency for the former and an increase for the latter. Experimental results were supported by band structure calculations whose results are consistent with the experiment concerning specific heat, resistivity, and magnetoresistance measurements. In particular, they reveal that of the s-electrons at the Fermi level, those of Al and Si have prevailing density of states and, thus predominantly contribute to the metallic conductivity. This also shows that the high residual resistivity of the materials studied is an intrinsic effect, not due to defects of the crystal structure.

Positive linear magnetoresistance effect in disordered L21B -type Mn2CoAl epitaxial films

The inverse Heusler alloy ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ is known as a spin gapless semiconductor (SGS). It is believed that the positive linear magnetoresistance (PLMR) effect observed at low temperatures indicates the quantum linear MR effect in the gapless electronic band structures near the Fermi level, i.e., evidence of the realization of a SGS [Ouardi et al., Phys. Rev. Lett. 110, 100401 (2013)]. For this reason, one presumes that the observation of the PLMR effect is indirect proof of the demonstration of $XA$-type ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ consisting of the inverse Heusler structure. In this paper, we observe the PLMR effect at 10 K in homogeneous and single-phase ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ epitaxial films grown by low-temperature molecular beam epitaxy at $100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. From anomalous x-ray diffraction measurements and analyses, we clarify that the $100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$-grown ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ epitaxial film is not composed of the $XA$-type structure but a disordered $L{2}_{1}B$-type structure including some amount of Mn(A site) $\ensuremath{\Leftrightarrow}\mathrm{Co}$(C site) swapping and Mn(B site) $\ensuremath{\Leftrightarrow}\mathrm{Al}$(D site) swapping. On the basis of first-principles density-functional theory calculations, we discuss the correlation between the PLMR effect observed at 10 K and the possible electronic band structure in the disordered $L{2}_{1}B$-type ${\mathrm{Mn}}_{2}\mathrm{CoAl}$.

Admittance spectroscopy of dopants implanted in silicon and impurity state-induced AC magnetoresistance effect

A silicon structure doped with Ga using ion implantation has been investigated by admittance spectroscopy. It has been established that the presence of the Ga impurity, along with the B one, in the silicon structure leads to the appearance of the second peak in the temperature dependence of the real part of the impedance (admittance). Moreover, switching-on a magnetic field parallel to the sample plane shifts the singularities in the temperature curve to the high-temperature region. This results in the manifestation of both the positive and negative magnetoresistance effect upon temperature and magnetic field variation. It has been found by the standard admittance spectroscopy analysis of the impedance data that the energy structure of the investigated sample includes two interfacial energy levels ES1(0) = 42 meV and ES2(0) = 69.4 meV. As expected, these energies are consistent with the energies of B and Ga dopants. In a magnetic field, these levels increase by 3 meV for B and 2 meV for Ga, which induces the magnetoresistance effect. It has been demonstrated that the interfacial state-induced magnetoresistance effect can be tuned by ion implantation and dopant selection.

Negative differential resistance and unsaturated magnetoresistance effects based on avalanche breakdown

The negative differential resistance (NDR) effect and magnetoresistance (MR) effect attract a lot of attention since they have been widely applied in fields such as circuit amplifiers and information storage, respectively. In general, the NDR and MR effects derive from different physical mechanisms, which makes it difficult to obtain both an NDR effect and a large unsaturated MR effect in a device based on the same physical mechanism. In this paper, the NDR and unsaturated MR effects were observed simultaneously in In/SiO2/p-Si/SiO2/In hetero junction devices, and their physical mechanisms were investigated. It is found that under zero magnetic field conditions, the NDR effect can be observed up to 25 K which is much higher than the temperature of NDR obtaining in intrinsic silicon. The NDR effect is significantly enhanced by applying a small magnetic field of 0.1 T. Meanwhile, the unsaturated positive MR effect is obtained when an external magnetic field is applied. Our analysis shows that the NDR effect and the unsaturated MR effect can be explained on the basis of the same physical mechanism of the avalanche breakdown in the In/SiO2/p-Si hetero junction. The NDR effect mainly results from complex effects containing the carrier injection after avalanche breakdown in the hetero junction, the reduction of the Schottky barrier height and the Joule heating effect. The large unsaturated MR effect derives from the suppression of the plasma due to avalanche breakdown by the applied magnetic field. The largest value of MR based on avalanche breakdown can reach about 530%@1 T at 10 K, and MR sensitivity can be as high as about 29.4 T−1 under 0.05 T.

Coupling of magnetoresistance switching and glassy magnetic state at the metal–insulator transition in Ruddlesden-Popper manganite Ca4Mn3O10

Electronic phase separation is a frequent ingredient of the physics of manganites for which the colossal magnetoresistance (CMR) phenomenon is at debate. We investigate the switchable magnetoresistance (MR) effect of the Ruddelsden-Popper (RP) oxide Ca4Mn3O10 (CMO), which shows a metal-to-insulator transition (MIT) at 70 K (TMIT). The static (DC) and dynamic (AC) magnetic susceptibilities of CMO indicate an unusual magnetic phase coexistence below the Néel temperature (TN) i.e., at TMIT, which we describe as a glassy magnetic behavior. Furthermore, by means of synchrotron X-ray diffraction, we observed a structural anomaly directly associated with the MIT. Hence, the onset of glassiness correlates with a slight structural distortion. Interestingly, CMO displays a large positive magneto-resistance (PMR) effect (up to 550% at 4 K) in the metallic state below TMIT, while a small negative magneto-resistance (NMR) effect (-3.5% at 112 K) in the insulating region above TMIT. At TMIT, the PMR switches to NMR behavior. PMR is probably induced via enhancing the exchange interaction by localization of itinerant moment resulting in formation of magnetic polarons, whereas NMR is possibly due to reduced electron-spin scattering. This appears to be a special type of magnetoresistance, different from regular observations, and may open a new avenue in searching for exotic phenomena in manganese-based CMR materials.

Tunable positive magnetoresistance and crossover from weak antilocalization to weak localization transition in half-Heusler compounds RPtBi (R = lanthanide)

We report the observation of large tunable positive magnetoresistance (MR) effects in magnetic lanthanide half-Heusler compounds RPtBi (R = Tb, Dy, Ho, Er, and Tm). The value of MR shows a great enhancement from 41% to 1740% by replacing R from Tb to Tm at 2 K under a magnetic field of 9 T. Moreover, a transition of MR from weak antilocalization to weak localization is clearly confirmed when the lanthanide R changed from Tm to Tb. We attribute this transition to gradually enhanced spin-dependent scattering in these magnetic lanthanide half-Heusler compounds RPtBi.

Memristive and magnetoresistance effects of SnSe&lt;sub&gt;2&lt;/sub&gt;

Memristor and magnetoresistance (MR) are widely used in the field of information storage. In recent years, SnSe&lt;sub&gt;2&lt;/sub&gt;, as an information storage material with both memristor and MR effects, has received a lot of attention of the researchers. It is of great significance to further explore its electrical transport mechanism. In this paper, the high-purity bulk SnSe&lt;sub&gt;2&lt;/sub&gt; samples are prepared by melting method together with spark plasma sintering. The &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curves are measured under different temperatures and magnetic fields. The memristive and MR effect of SnSe&lt;sub&gt;2&lt;/sub&gt; are systematically investigated. After the memristive characteristics are excluded from interfacial junction effect, phase transition and conductive wire channels, the memristive effect at different temperatures is attributed to the space charge limiting current effect under defect control. Under low electric field conditions, the internal carrier concentration of material is much higher than the injected carrier concentration and the &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curve obeys ohmic conduction. When the voltage increases to the switching voltage &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;on&lt;/sub&gt;, the internal defects of the material are filled with the injected carriers as the transport time of the injected carrier is less than the dielectric relaxation time, and the &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curves deviate from ohmic conductivity. When the voltage reaches the transition voltage &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;TFL&lt;/sub&gt;, the injected carrier increases exponentially, and the &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curve presents negative differential phenomenon. Finally, the space charge inside the material will limit the further injection of external carriers, and the &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curve follows the Child law. As the temperature decreases to 10 K, the memristive phenomenon weakens because a large number of defects for accepting the injected carriers are reduced due to the decrease of impurity ionization at low temperatures. At the same time, the sample exhibits a large negative MR at 10 K and 100 K. When impurity scattering predominates, the electrons will be subjected to multiple scattering by the impurities, resulting in localization of carriers. The negative MR effect is related to the inhibition of the carrier localization by the magnetic field. In our work, a large negative MR of about –37% at 0.6 T and 10 K are obtained, which is likely to originate from the disordered distribution of Se vacancy in the material. With the increase of temperature, the scattering mechanism gradually evolves from the impurity scattering into the lattice scattering, and the negative MR effect gradually develops into the positive MR effect.

Anomalous Angle-Dependent Magnetotransport Properties of Single InAs Nanowires.

We study the magnetotransport properties of single InAs nanowires grown by selective-area metal-organic vapor-phase epitaxy. The semiconducting InAs nanowires exhibit a large positive ordinary magnetoresistance effect. However, a deviation from the corresponding quadratic behavior is observed for an orientation of the applied magnetic field perpendicular to the nanowire axis. This additional contribution to the magnetoresistance can be explained by diffuse boundary scattering of free carriers in the InAs nanowire and results in a reduction of the charge carrier mobility. As a consequence, angle-dependent magnetotransport measurements reveal a highly anomalous behavior. Numerical simulations have been conducted to further investigate the effect of classical boundary scattering in the nanowires. On the basis of the numerical simulations, an empirical description is derived, which yields excellent agreement with the experimental data and allows one to quantify the contribution of boundary scattering to the magnetoresistance effect.

Semiconductor-to-metallic spin-filtering and positive and negative magnetoresistance effects in C3N with nickel electrodes

Abstract Carbon-based spintronic devices have attracted extensive attention recently because they have long spin relaxation times and lengths. As a new member of the carbon material family, C3N may have great application prospects in the field of spintronics. Though a previous work proposed the model of spintronic devices based on bare C3N nanoribbons, these devices are difficult to implement experimentally. This is due to the fact that edge reconstruction will occur in the bare C3N nanoribbons. In this work, we systematically investigate the spin-polarized properties of 2D C3N sheet and its corresponding hydrogen-passivated nanoribbons with ferromagnetic Ni electrodes. Semiconductor-to-metallic spin-filtering and positive magnetoresistance effect are observed in 2D C3N sheet stacked on top of the Ni electrodes. Especially impressive is the enhancement of spin-polarization efficiency and the emergence of negative magnetoresistance effect after the 2D C3N sheet is converted into hydrogen-passivated nanoribbons. These results indicate that C3N has strong potential for nanoscale spintronics applications.

Experimental verification of the origin of positive linear magnetoresistance in CoFe(V1−xMnx)Si Heusler alloys

The origin of the positive linear magnetoresistance (PLMR) effect at low temperatures in $\mathrm{CoFe}({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x})\mathrm{Si}$ Heusler alloys is discussed. From anomalous dispersion x-ray diffraction measurements, we can clarify that the crystal structure of the epitaxial CoFeVSi films grown by molecular beam epitaxy (MBE) is $L{2}_{1}\mathrm{B}$ type. By employing the substitution of Mn for V in the CoFeVSi film in MBE conditions, we experimentally demonstrate homogeneous $\mathrm{CoFe}({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x})\mathrm{Si}$ films and systematic control of the magnetic moments. The PLMR effect gradually decreases with increasing Mn contents but it can be seen even for $\mathrm{CoFe}({\mathrm{V}}_{0.5}{\mathrm{Mn}}_{0.5})\mathrm{Si}$. We note that the PLMR effect disappears for $\mathrm{CoFe}({\mathrm{V}}_{0.25}{\mathrm{Mn}}_{0.75})\mathrm{Si}$. According to theoretical calculations of the electronic band structures of $\mathrm{CoFe}({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x})\mathrm{Si}$, the gaplesslike electronic band structure due to the V-$d$ state in the minority spins of CoFeVSi is varied to the band gap by increasing the substitution of Mn. This means that the electronic band structure near the Fermi level in $\mathrm{CoFe}({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x})\mathrm{Si}$ can be experimentally controlled by substituting Mn for V. From these considerations, we can conclude that the PLMR effect in $\mathrm{CoFe}({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x})\mathrm{Si}$ is caused by the presence of the V state-induced gaplesslike structure in the minority spin electronic band near the Fermi level.

Selective-area growth and transport properties of MnAs/InAs heterojunction nanowires

Abstract

Anisotropic magnetoresistance behaviors in the layered ferromagnetic Cr2Ge2Te6

The ferromagnetic semi-conductive Cr2Ge2Te6 with van der Waals bonded layers is prominent for characteristics of easy exfoliation and tunablility, which make it a potential candidate for spintronic application. Although the magnetism has been investigated intensively, the transport property is rarely studied, which is very important for the realization of its application. In this work, the electronic transport of Cr2Ge2Te6 is investigated under external magnetic field with variable directions. The angle-dependent resistance shows absolute isotropic property in the ab-plane. However, it displays anisotropic characteristic when the magnetic field is rotated from the ab-plane to the c-axis. The resistance reaches the maximum when H//c while it is the minimum when H//ab. Moreover, the field-dependent resistance of Cr2Ge2Te6 shows a positive magnetoresistance (MR) effect when H//c but a negative one when H//ab. We suggest that the anisotropic MR behaviors of Cr2Ge2Te6 can be understood theoretically within the framework based on the exchange interaction between the carriers and local magnetic moments.

Large thermal Hall effect in α−RuCl3 : Evidence for heat transport by Kitaev-Heisenberg paramagnons

The honeycomb Kitaev model in a magnetic field is a source of a topological quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. We present experimental results for the thermal Hall effect of the material $\alpha$-RuCl$_{3}$ which recently emerged as a prime candidate for realizing such physics. At temperatures above long-range magnetic ordering $T\gtrsim T_N\approx8$ K, we observe with an applied magnetic field $B$ perpendicular to the honeycomb layers a sizeable positive transversal heat conductivity $\kappa_{xy}$ which increases linearly with $B$. Upon raising the temperature, $\kappa_{xy}(T)$ increases strongly, exhibits a broad maximum at around 30 K, and eventually becomes negligible at $T\gtrsim 125$ K. Remarkably, the longitudinal heat conductivity $\kappa_{xx}(T)$ exhibits a sizeable positive thermal magnetoresistance effect. Thus, our findings provide clear-cut evidence for longitudinal and transverse magnetic heat transport and underpin the unconventional nature of the quasiparticles in the paramagnetic phase of $\alpha$-RuCl$_{3}$.

Observation of superconductivity in the pressurized Weyl-semimetal candidate TaIrTe4

Here we report the observation of superconductivity in pressurized type-II Weyl semimetal (WSM) candidate TaIrTe4 by means of complementary high-pressure transport and synchrotron X-ray diffraction measurements. We find that TaIrTe4 shows superconductivity with transition temperature (TC) of 0.57 K at the pressure of ~23.8 GPa. Then, the TC value increases with pressure and reaches ~2.1 K at 65.7 GPa. In situ high-pressure Hall coefficient (RH) measurements at low temperatures demonstrate that the positive RH increases with pressure until the critical pressure of the superconducting transition is reached, but starts to decrease upon further increasing pressure. Above the critical pressure, the positive magnetoresistance effect disappears simultaneously. Our high pressure X-ray diffraction measurements reveal that, at around the critical pressure the lattice of the TaIrTe4 sample is distorted by the application of pressure and its volume is reduced by ~19.2%, the value of which is predicted to result in the change of the electronic structure significantly. We propose that the pressure-induced distortion in TaIrTe4 is responsible for the change of topology of Fermi surface and such a change favors the emergence of superconductivity. Our results clearly demonstrate the correlation among the lattice distortion, topological physics and superconductivity in the WSM.

Positive and negative magnetoresistances in Co/Cu/Ni spin-valves

We studied the magnetoresistance effects in Co/Cu/Ni spin-valves with magnetic fields along different in-plane directions. When the magnetic easy axis directions of Co and Ni layers are parallel, only typical positive magnetoresistance effects were discovered. Interestingly, a magnetoresistance sign inverse was discovered in the spin-valve with different magnetic easy axis directions between Co and Ni layers. This anomalous behavior may be related to the multi-step magnetization reversal processes in these materials.

Large Magnetoresistance and Electrical Transport Properties in Reduced Graphene Oxide Thin Film

We report a systematic study of room temperature large positive and negative magnetoresistance (MR) in the reduced graphene oxide (RGO) thin-film devices grown by pulsed-laser deposition (PLD) at high and low applied magnetic fields, respectively. Raman spectroscopy, X-ray photoelectron spectroscopy, and electrical measurements on the RGO films help to explain the observed MR properties in the device. The temperature-dependent (5–400 K) electrical characterization of the thin films shows two distinct transport regimes: at low temperature, it follows 2-D Efros-Shoklovoski variable range hopping (VRH) transport mechanism and above 200 K, the device shows Arrhenius-like transport behavior. The crossover from VRH transport to Arrhenius transport is due to shortening in the characteristic lengths in the disordered 2-D system. We interpret the source of negative MR by vacancy and disorder-induced magnetic moments and the diffuse scattering at crystallite boundaries. At the high applied magnetic field, the lifting in degeneracy due to the Lorentz force explains the large positive MR effect. The highest value of the measured MR (160%) is surprisingly high for a non-magnetic material at room temperature, which can be attributed to the greater inhomogeneity in the PLD grown wafer-scale RGO thin films.

Properties of La1.85Sr0.15CuO4/La0.7Sr0.3MnO3 hybride nanowire networks prepared by electrospinning

Abstract We prepared La 1.85 Sr 0.15 CuO 4 / La 0.7 Sr 0.3 MnO 3 nanowire network hybrid systems by means of electrospinning. From observation by scanning electron microscopy, the average diameter of the nanowires is around 220 nm and the average length can reach over 50 μ m. The randomly aligned La 1.85 Sr 0.15 CuO 4 and La 0.7 Sr 0.3 MnO 3 nanowires exhibit numerous interconnects among each other and form a complicated, non woven hybrid network system. The nanowires are polycrystalline with a grain size at around 30 nm as confirmed by transmission electron microscopy. According to four-probe electrical transportation and magnetization measurements, superconductivity of the sample is suppressed and a positive magnetoresistance effect is observed. Measurements of M(T) and M(H) were carried out as well, revealing the soft magnetic character of the hybrid nanowire network samples.